Flammable gases are generally used as energy sources but some situations may require the use of gas flares for their destruction, for instance in the event of a production surplus or an unexpected shutdown of an equipment in which a flammable gas is normally burned to generate heat. Other situations exist. Some flammable gases are byproducts of natural or industrial processes where the flammable gas source cannot be stopped and/or be easily controlled, and that the flammable gases cannot be stored for a later use. Thus, in case of a surplus of flammable gases or an unexpected equipment shutdown in such context, a gas flare is an alternative to releasing the flammable gases directly into the atmosphere.
One example of a flammable gas source that cannot be stopped and/or be easily controlled is a landfill site. In a landfill site, organic matters contained in the waste slowly decay over time using a natural process and generate a gas stream containing methane gas (CH4). Methane gas is a flammable gas and is mixed with other flammable and non-flammable gases in varying proportions when coming out of the landfill site. Methane gas is a valuable source of energy but is also a greenhouse gas if released directly into the atmosphere. Thus, if the methane gas contained in a gas stream coming out of a landfill site cannot be readily used or stored, it should be destroyed by combustion in a gas flare. Gas streams containing methane gas can also be created by other processes, for instance in an anaerobic digester. Many other situations and contexts exist.
A waste gas stream could also be a flammable gas or a mixture of flammable gases that is simply unusable for some reason and for which the destruction is required. This flammable gas or mixture can even represent 100% or close to 100% of the total waste gas content.
Waste-to-energy projects are systems designed for transforming at least a portion of the flammable gas or gases contained in gas streams into useful energy, for instance heat energy. They receive gas streams from sources such as landfill sites and anaerobic digesters, thus from sources that contain waste materials. For this reason, these gas sources can be referred to as waste gas sources and the gases flowing therefrom can be referred to as waste gas streams. Capturing waste gas streams offers significant environmental and economic benefits when used in a waste-to-energy project since the waste gas streams would otherwise be released into the atmosphere or be simply burned off in gas flares on a continuous basis.
Various factors may affect the proportion of methane gas fraction in waste gas streams coming, for instance, from a landfill site. The flow rate of the collected gases can also vary over time. In a waste-to-energy project constructed next to a landfill site, it may happen that the waste gas stream from the landfill site is generated in excess to what the waste-to-energy system can consume. Still, the waste-to-energy project can also be abruptly stopped in an unplanned manner. These are examples of situations where having a gas flare associated with a waste-to-energy project is required for suitably destroying the flammable gases in the waste gas stream. Many other situations exist as well.
One of the challenges in the design of gas flares, particularly in the context of waste-to-energy projects, is the unpredictability in the need of operating them and the usually long standby periods. Waste-to-energy projects can run continuously for months without the need of operating an associated gas flare. As a result, the gas flare can be difficult to restart after a prolonged standby period. Rain water and snow accumulations inside the flare stack can also prevent the gas flare from starting when needed. Other factors and complications exist, all of which can hinder the overall efficiency and operation of the gas flares over time. Existing gas flares are not well adapted to relatively long standby period, especially under inclement weather conditions like heavy rain or freezing temperatures, to name just a few. Extensive maintenance operations by on-site technicians can be required simply to restart gas flare.
Another challenge in the design of gas flares is that the destruction of methane gas or of other flammable gases present in waste gas streams using a gas flare is generally highly regulated. For example, the residence time of the flue gas in the combustion chamber and its temperature must often meet certain minimum values to insure that flammable gases have been destroyed in the gas flare with an efficiency of at least 98%. Minimizing the temperature during the destruction of the flammable gas is also often desirable so as to minimize nitrogen oxides (NOx) formations in the flare stack chamber at high temperatures. Having a low-NOx system decreases air pollution.
As the flammable gas fraction in a waste gas stream often varies over time, it may happen that the flammable gas fraction falls down to the point where the waste gas stream can no longer be used as a source of energy at the waste-to-energy project. In a waste gas stream coming from a landfill site, the methane gas fraction is generally about 25% to 65% in weight of the total waste gas stream. A very relatively small proportion of flammable gas will increase the difficulty of sustaining the flame and if the proportion is too low, no flame can be generated.
The flow rate of the waste gas stream itself can also vary anywhere from 0 to 100% of the gas flare capacity. Gas flares must be capable of handling up to the maximum flow rate of the waste gas stream that can be produced by the waste gas source. However, most gas flares have a relatively low turndown ratio, such as 3:1. The turndown ratio is the ratio between the maximum and minimum flow rates of the waste gas stream that can be processed by the gas flare. Having a low turndown ratio restricts the possibility of destroying the flammable gas through combustion when the flow rate is relatively small because the burner arrangement of the gas flare would be too large.
Accordingly, there is still room for many improvements in this area of technology.